[mellophone]

I recently purchased a Tamron 17-50mm f/2.8 for my K20D but it hasn't lived up to my expectations - the lens continually switches between autofocus and manual focus even though I've set it to AF-C or AF-S. Most of the time it also won't let me select a focus point (as in it will switch between the selected point and the centre point by itself).

I've tried cleaning the contacts on both the lens and camera but with no effect. Is the lens faulty or am I doing something idiotic?

[Adam]

The only time this even happened to me with a lens was when it wasn't mounted correctly (i.e. the screwdrive lever wasn't connecting properly), so I'd say that this sounds like a lens issue.

[Ash]

But the switching focus points sounds like a camera issue. Check your focus mode dial to ensure it is clicked onto a mode correctly, then test out your point selections with the 4 way controller.

[mellophone]

After playing around some more, I found that I can temporarily fix the problems by applying pressure to the lens (especially around the focus ring) - this makes me think its an issue with the contacts although externally they appear normal. I can also hear the screwdrive lever sometimes clicking out of place even though the lens appears to be mounted correctly.

I don't think its the body body though because my two other autofocus lenses work without a hitch.

So my new plan is to send this lens back and get myself a brand new 17-70 sigma instead - hopefully I won't miss the constant f/2.8 too much. At least I'll get some macro-like capability.